Approximately 20-25% of patients with acute myeloid leukemia (AML) are cured following intensive chemotherapy. Interleukin (IL)-2, administered to AML patients in remission, has been investigated as a means of potentially reducing relapse and improving outcome. IL-2 in vivo expands and activates natural killer (NK) cells, which do not require the recognition of leukemia-specific antigens for killing of leukemic cells. Recent understanding of the importance of NK cell receptors (NKR) for the recognition and lysis of target cells, however, suggests that not all patients are likely to benefit from this immunotherapy strategy. Recognition and lysis of leukemic cells by NK cells is regulated by a balance of activating and inhibitory surface receptors that interact with specific MHC class I and class I-like ligands on target cells. Our preliminary data indeed suggests that primary AML cells, and not only cell lines, express ligands to the activating NKG2D receptor of NK cells. Furthermore, that expression of these ligands by AML is heterogeneous, in keeping with the molecular heterogeneity of the disease. We hypothesize that only leukemic cells that express high levels of activating ligands and/or low levels of inhibitory ligands are susceptible to autologous NK cell lysis, and that patients with such leukemic blasts are those most likely to benefit from IL-2 therapy. To investigate this hypothesis, we propose to perform correlative studies on samples procured from AML patients enrolled on Cancer and Leukemia Group B studies of IL-2 immunotherapy, investigating the relationship between the expression of known important NK cell activating and inhibitory ligands on leukemic blasts, in vitro susceptibility of AML cells to autologous IL-2 expanded NK cells, and clinical outcome. Specifically, we aim to 1) Correlate the in vitro lysis of pre-treatment AML blasts by IL-2 in vivo expanded NK cells with relapse-free survival (RFS) following IL-2 therapy, 2) Correlate the expression of inhibitory (MHC class I) and activating ligands on AML blasts with RFS, and 3) Compare the susceptibility to NK cell lysis of leukemic blasts obtained at diagnosis and at relapse. The results of our studies may provide a means of identifying which subset of AML cases are susceptible to IL-2 therapy, and therefore, may ultimately assist in the better selection of patients for NK cell-based therapies.